The present application claims priority from Japanese Patent Application No. 2017-098572, filed May 17, 2017, which is incorporated herein by reference.
The present invention relates to a portable working machine such as a bush cutter and a chain saw and, more particularly, to a portable working machine including an engine with a carburetor and a fuel supply control method thereof.
A portable working machine such as a bush cutter, a chain saw, a power blower, and a trimmer employs an internal combustion engine, for example, a two-stroke engine, as a drive source and employs a carburetor. Obviously, the portable working machine can employ a four-stroke engine as the driving source.
The carburetor has an intra-carburetor air passage through which air filtered by an air cleaner passes and has a fuel discharge part discharging fuel to the intra-carburetor air passage. The fuel discharge part is supplied with fuel from a fuel source through a fuel supply passage. The carburetor allows the air passing through the intra-carburetor air passage to suck out the fuel from the fuel discharge part and thereby generates an air-fuel mixture.
Before shipping portable working machines, a manufacturer performs adjustment to optimize an amount of the fuel (fuel supply) discharged through the fuel discharge part. The fuel supply passage is generally provided with a manual needle valve, and the fuel supply is adjusted by adjusting the valve opening degree of this needle valve. With this adjustment, individual differences of working machines to be shipped are eliminated, and the operation of the working machines to be shipped is optimized.
However, the environment (e.g., atmospheric pressure, temperature) is not the same when users use the working machines. A type of fuel used is also different. Therefore, it is necessary for users to readjust the fuel supply. This readjustment is not easy for many users.
Patent Document 1 discloses a carburetor with a solenoid valve disposed in a fuel supply passage and a portable working machine incorporating the same. The valve opening degree of the solenoid valve is electronically controlled by using a rotation speed sensor detecting the engine rotation speed. Specifically, in the fuel supply control disclosed in Patent Document 1, after completion of a warming-up operation, the valve opening degree of the solenoid valve is feedback-controlled while detecting the engine rotation speed with the rotation speed sensor such that a preset target rotation speed is achieved without a load and with wide open throttle (“full throttle”). The valve opening degree of the solenoid valve capable of achieving the target rotation speed without a load is stored in the memory.
When a work is performed by using the working machine, the engine rotation speed decreases due to a load acting on the engine. In Patent Document 1, the valve opening degree of the solenoid valve is corrected based on a difference between the target rotation speed without a load and the engine rotation speed detected under a load. This correction amount is obtained from a map prepared in advance. In the map, a correction amount corresponding to each difference is predefined by using the engine rotation speed corresponding to the difference as a parameter.
Patent Document 1: Japanese Laid-Open Patent Publication 2013-204552 (counterpart US2013/0255629A1)
As described above, in Patent Document 1, the fuel supply control is carried out based on the engine rotation speed detected by the rotation speed sensor. A work with a chain saw is performed with wide open throttle.
On the other hand, a brush cutter is not limited to the operation with wide open throttle (full throttle). The work may be performed with a partial throttle opening degree depending on a state of grass to be cut. The cutting blades of the brush cutter include metal blades and plastic blades (nylon cords) immediately worn when used, and the nylon cords and the metal blades are selectively attached to the brush cutter for use. Since the magnitude of the load acting on the engine differs between the nylon cords and the metal blades, the rotation speed is different when the throttle valve is wide open. With the wide-open throttle valve (full throttle), the metal blades provide a rotation speed exceeding 10,000 rpm, while long nylon cords result in 6,000 rpm, for example. Additionally, the nylon cords change in length due to wear during use. Therefore, when the nylon cords are used, the load acting on the engine varies every moment during work.
Although the fuel supply control disclosed in Patent Document 1 may be able to optimize the fuel supply under limited conditions, the optimization of the fuel supply is practically impossible in the fuel supply control based on the rotational speed, particularly, in a partial operation or a work using nylon cords.